The invention relates to wavelength selective opto-mechanical switching devices and methods of manufacture thereof. More specifically the invention relates to a micro-mechanical structure that employs ring resonator or disc resonator structures to implement multi-channel wavelength selective optical add/drop multiplexers.
In multi-channel communication systems, it is desirable to provide a selective coupling mechanism. This allows signal channels to be added to lines and dropped from lines. It may also be desirable to provide for the splitting of a signal channel among multiple lines. To realize a large communications network, it may be desirable to provide arrays of such add/drop couplers and signal splitters and numerous separate communications lines for the various channels. Desirably these arrays may be packaged together as an integrated multiplexer/demultiplexer device, which is compact, has low loss characteristics, and creates little cross talk between channels.
Additionally, a multi-channel communication network desirably contains a plurality of controllable switching means to allow rerouting of data signals within the network. This allows the flow of signal channels to be interactively altered during operation.
It may be desirable for the functions of the add/drop couplers and signal splitters to be combined with controllable switching functions. This situation may be accomplished through the use of controllable add/drop switches and variable signal splitters. An array of such controllable add/drop switches and variable signal splitters may be seen as a simple, rapidly reconfigurable integrated multiplexer/demultiplexer device. Such a device may allow the data flow patterns within a communications network to be almost instantaneously changed.
Fiber optics communication systems provide a method of streamlining the communications lines. A single optical fiber can carry a number of separate communication channels, each channel operating bi-directionally at a different wavelength. The larger the number of wavelengths that may operate simultaneously within a fiber, the greater the capacity of the fiber.
Several factors determine what the maximum number of wavelengths operating in a single fiber can be. The first factor is the spectral range over which the fiber has a low enough loss and a high enough confinement factor to make transmission practical. The second factor is the spectral width of the laser sources used for the communications system and how much these spectra are broadened during transmission through the system. A third factor is the precision with which the channels may be separated from one another.
Providing a compact, highly discriminating, low loss multiplexing system for multi-wavelength optical communication systems has been very challenging. Some approaches that have been tried include; dynamically configurable gratings, prisms, or filters. While these means are perfectly adequate for many multiplexing systems, they suffer the drawback that the number of channels that the system may handle is limited because the wavelength dispersion of the demultiplexing means is not adequate to separate very closely spaced channels in devices of reasonable dimensions. Additionally, these approaches are not easily amenable to miniaturization.
Other wavelength multiplexers have been described in the literature. For example, frequency selective coupling means, i.e., evanescent couplers, have been proposed as an alternative to means that rely on dispersive properties of the multiplexer components. An evanescent coupler, in its simplest embodiment, uses at least two optical waveguides in such close proximity that the propagating mode of the second waveguide is within the exponentially decaying evanescent portion of the propagating mode of the first waveguide. The overlap couples optical energy into the second waveguide if the propagation constants, k, in the two guides are equal. If the values of k are equal at only a single frequency, only energy at that frequency is coupled while energy at other frequencies remains in the first guide. H. F. Taylor describes such a frequency selective coupling scheme in Optics Communications, 8, pp. 421-425, August 1973. The couplers described used optical coupling between two non-identical waveguides to couple the single optical frequency for which the propagation constants in the two guides are equal. Optical bandwidths of approximately several tens of Angstroms may be achieved in 1 cm long couplers thus theoretically permitting about 100 optical channels. These systems, however, are not readily controllable.
Micro-ring resonator couplers have been proposed for use in optical communications systems. One such micro-ring system is described in, S. T. Chu, et al., xe2x80x9cAn Eight-Channel Add-Drop Filter Using Vertically Coupled Micro-ring Resonators over a Cross Gridxe2x80x9d, IEEE Photonics Technology Letters, Vol 11, No 6, June 1999. In this work the authors describe the application of micro-ring resonators to add/drop filters. The add/drop filters are formed with a first layer containing a pair of waveguides which form a cross. A micro-ring resonator is disposed directly on top of the waveguides, near their intersection. The edge of the micro-ring resonator overlaps both waveguides allowing optical signals to be coupled from one waveguide into the ring and then from the ring into the other waveguide. This article is hereby incorporated herein by reference for its teaching on micro-ring resonators.
An array of micro-ring add/drop filters, as described by Chu, may provide static multiplexing and demultiplexing of numerous optical communication channels operating with narrow wavelength bands. This system may also be integrated into a compact package. The system described by Chu does not provide for dynamic reconfiguration of the filter array. Therefore, although this system contains many desirable advantages as a multi-wavelength optical multiplexer/demultiplexer for optical communication signals, it does not provide a fully dynamic integrated switching filter array for optical communications networks. Additionally, the intersecting of the waveguides in a filter array designed according to Chu may lead to scattering of the optical signals at the junctions, causing signal loss and/or undesired coupling (i.e. crosstalk) between the waveguides.
One embodiment of the present invention is an exemplary wavelength selective optical coupling device. The exemplary wavelength selective optical coupling device is formed on a substrate. In alternative embodiments of the present invention the substrate may include control circuitry.
A primary waveguide and a secondary waveguide are disposed on the top surface of the substrate separate from one another. The waveguides are each adapted to transmit a plurality of different wavelengths of light and each includes a portion for optically coupling to a ring or disc resonator. The resonator includes a dielectric member which extends parallel to the top surface of the substrate and overlaps, without contacting, the coupling portions of the two waveguides. The ring or disc resonator is sized to resonate at a subset of resonant wavelengths of the wavelengths of light transmitted in the waveguides.
In alternative embodiments of the present invention the resonator may further include an electrical heating element disposed on the upper surface of the dielectric member and means for applying an electric current to the electrical heating element, whereby the subset of resonant wavelengths of the ring or disc resonator may be tuned by the level of current applied to the heating element from control circuitry in the substrate.
Resonator coupling means for coupling the ring or disc resonator to the top surface of the substrate are also provided. In alternative embodiments of the present invention the resonator coupling means may include a bridge coupled to the top surface of the substrate and electrically coupled to control circuitry in the substrate. Control circuitry may be adapted to provide a waveguide coupling signal and the bridge adapted to deform in response to the waveguide coupling signal. This deformation of the bridge may be used to translate the resonator between a waveguide decoupled position and a waveguide coupled position. The ring or disc resonator is optically coupled to the primary waveguide and the secondary waveguide in the waveguide coupled position and substantially decoupled from at least one of the primary waveguides and the secondary waveguide in the waveguide decoupled position.
Another embodiment of the present invention is a multi-wavelength optical multiplexer formed on a single substrate containing coupling control circuitry adapted to provide a plurality of waveguide coupling signals. A plurality of waveguides are disposed on the top surface of the substrate, each waveguide separated from the remaining waveguides. The waveguides are adapted to transmit the plurality of wavelengths of light. Also disposed on the substrate are a plurality of switchable wavelength selective optical couplers designed in accordance with the previously described embodiments of the present invention.
Another embodiment of the present invention is a method of manufacturing a wavelength selective waveguide coupling device according to the previously described embodiments of the present invention.
Another embodiment of the present invention is a method for encoding information as a narrow wavelength band digital optical signal. The method uses an integrated optical component including an optical source optically coupled to a first waveguide, a second waveguide, a ring or disc resonator sized to resonate in a narrow resonator wavelength band, a resonator translating means adapted to translate the resonator between a coupled position and a decoupled position, and control circuitry electrically coupled to the resonator coupling means. The control circuitry is adapted to form a digital electric signal in response to information provided to the integrated optical component.
The method includes several steps. The first is to operate the optical source, thereby providing optical radiation to the first waveguide. This optical radiation is characterized by a substantially constant amplitude and a source wavelength band which includes the narrow resonator wavelength band. The next step is to provide information to the control circuitry, so that it may be formed into a digital electric signal. This digital electric signal is then provided to the resonator translating means causing it to translate the ring or disc resonator between the coupled position and the decoupled position. Optical radiation in the narrow resonator wavelength band is then transferred from the first waveguide to the second waveguide only when the ring or disc resonator is in the coupled position, thus encoding the information as a narrow wavelength band digital optical signal traveling in the second waveguide.
One feature of several embodiments of the present invention is the use of micro electrical mechanical systems (MEMs) technology. This nanotechnology allows these embodiments provide high-speed, wavelength selective switching waveguide couplers, that may be integrated onto single integrated circuits, which may include the control and/or monitoring circuitry, as well as optical sources. Additionally, the degree of switching at each coupler may be accurately controlled, allowing signal splitting as well as add/drop switching.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.